1. Field of the Invention
This invention relates generally to electronic imaging devices and more particularly to a system and method for generating timing signals in an electronic imaging device.
2. Description of the Background Art
The efficient operation of electronic imaging devices provides computer users with new and effective ways to capture and process various types of image data. Electronic imaging devices typically include special electronic image sensors that convert a selected image into electronic data. These electronic image sensors conventionally include a series of discrete picture elements (or pixels) which convert light reflected from a photographic target into electrical energy that is then shifted out of the image sensor device. Due to the complexity of the high-speed process involved in shifting captured image data out of the image sensor, electronic imaging devices require a series of precise timing pulses to successfully gate the image data from the image sensor. Electronic imaging devices (such as digital cameras) typically include a timing generator device that effectively generates and provides the precise timing pulses to the electronic image sensor.
Referring now to FIG. 1, a timing diagram of frame timing for a conventional digital camera is shown. Electronic imaging devices using image sensors, such as charge-coupled devices (CCDs), were traditionally used in video applications. The National Television Standard Committee (NTSC) video used in North America is typically displayed at 60 fields per second, and interlaced at 2 fields per video frame. Thus standard video is typically implemented at a fixed rate of 30 frames per second. The Sequential Couleur Avec Memoire (SECAM) video used in France and the Phase Alternating Line (PAL) video used in other parts of Europe is typically displayed at 50 fields per second, for a fixed rate of 25 frames per second. Hereinafter video in compliance with the NTSC standard will be addressed, but it is to be understood that the discussion will also apply to SECAM and PAL.
When electronic imaging devices were initially used for purposes other than video, the existing circuitry was adapted for this new use. Hence, conventional electronic imaging devices typically use a fixed frame rate of 30 frames per second. FIG. 1 shows a series of fixed time period frames each having a length of {fraction (1/30)}th of a second, which corresponds to a frame rate of 30 frames per second.
Image sensors such as CCDs operate as an array of photodiodes. Each photodiode generates electrons from the incident light photons. The electrons are stored in a corresponding capacitive element whose voltage output is proportionate to the stored charge. Prior to capturing an image, the image sensor is kept in a discharged state. When the image sensor is set to capture an image, the charge is allowed to build up in each element of the array for the period of the exposure in proportion to the intensity of the incident light. The voltage resulting from this charge may then be read from each element of the array and subsequently changed into digital form.
This sequence of events is shown in the timing diagram of FIG. 1. The fixed frame time is set to {fraction (1/30)}th of a second. The transfer substrate charge time period, called the Xsub time, is the period in which the image sensor is kept in a discharged state. The Xsub time begins simultaneously with the beginning of the frame time 70. At the end of the Xsub time 72 the exposure time begins. When the exposure is complete, at the end of the exposure time 74, the charge from the array of photodiodes is transferred simultaneously into a parallel array of analog storage locations, and then the photodiode array is discharged during the next Xsub time. As the next Xsub time begins, at the beginning of the next frame time 74, the image analog signals in the parallel array of analog storage locations are separately converted to digital form and shifted into a digital input buffer during the data shift time. When the data shifting is complete, at time 76, the digital processing circuits are ready to convert another exposure. This process of data shifting must be complete prior to the beginning of the next frame, which initiates analog transferring of the charges from the photodiode array to the parallel array of analog storage elements. Otherwise the next exposure""s charges would overwrite the analog data from the previous exposure.
There are at least two significant problems that arise from the frame time being fixed at 30 frames per second. A first problem is that in low light conditions the exposure may ideally need to be longer than {fraction (1/30)}th of a second (a conventional frame period). A second problem is that the digital camera may not be able to complete data shifting prior to the beginning of the next frame period. Since the frame period is fixed, these situations require the skipping of frames. Skipping every other frame allows for an effective frame time of {fraction (1/15)}th of a second, and skipping two out of every three frames allows for an effective frame time of {fraction (1/10)}th second.
Frame skipping allows both exposure times and data shifting times to be longer than the fixed frame time of {fraction (1/30)}th of a second. The drawback of frame skipping is that it may cause unsteady motion in the viewfinder of the electronic imaging device if the rate is too low. Human perception of image motion becomes sensitive to frame rate changes in the range below approximately 20 frames per second. As an example, one of the reasons that Super 8 mm motion picture film quickly supplanted standard 8 mm film is that the frame rate was increased from 16 frames per second to 18 frames per second. This numerically small difference is enough to make the resulting motions appear much more smooth and lifelike. Current theatrical motion picture film uses 24 frames per second, and video, as mentioned above, typically uses 30 frames per second.
Electronic image sensor devices are currently evolving to become increasingly more complex and thus require timing generators with more advanced capabilities and greater flexibility. Therefore, an improved system and method are needed for generating timing signals in an electronic imaging device.
In accordance with the present invention, a system and method are disclosed for generating variable-length timing signals in an electronic imaging device. In the preferred embodiment of the present invention, a digital camera device includes an electronic image sensor that requires a complex set of timing signals to effectively capture image data. The digital camera therefore also includes timing source circuitry which generates a set of precise timing signals for a variable-length frame time necessary to control and synchronize the electronic image sensor within the digital camera.
Three timing components are necessary to control and synchronize the electronic image sensor: a frame time, an exposure time, and a transfer substrate charge (Xsub) time. Any two of these time periods may be selected as independent variables, the third time period being dependent on the selected two time periods. In the preferred embodiment, the present invention includes an Xsub time source circuit and a frame time source circuit. The Xsub time source circuit and frame time source circuit each contain a downcounter. These devices decrement internal binary values every time a clock pulse is applied. When loaded with a specified binary value, they decrement to the value zero after a time period equal to the specified value multiplied by the clock pulse period. The points in time when the downcounters reach zero are thus available as precise and programmable timing references.
In the preferred embodiment of the present invention, the Xsub time downcounter and frame time downcounter are clocked by the horizontal drive (HD) pulses. These HD pulses are sufficiently short in duration to give a high resolution to the programmed time intervals. Both the Xsub time downcounter and frame time downcounter are loaded from registers under software control. The contents of these registers determine the length of the Xsub time and frame time, and when the software changes the values contained in the registers the next Xsub time and frame time will reflect the changed values. The contents of these registers may be selected to minimize the frame time, consistent with the operation of the electronic image sensor, which will maximize the frame rate. This maximized frame rate allows the smoothest perceived motion possible in the digital camera""s viewfinder. The present invention thus allows improved operator convenience consistent with the timing requirements of the electronic image sensor of the digital camera.